1. Field of the Invention
This invention relates to a device for measuring thermal power exchanged between a feed line and a return line of a thermal arrangement, and more particularly to a device in which energy heat is transmitted by means of a flow of either a liquid medium, a gaseous medium or a powdery medium, preferably for continuously measuring thermal power of heaters, heat exchangers, radiators, refrigerators, sanitary warm-water appliances, air-conditioning plants and other heating appliances. The device is supplemented with a meter and an integrator in the electronic part. It can either be used as a meter of consumed heat energy or of supplied heat energy and/or a continuously operating mass flow meter for liquids, gases and powdery materials, when there exists a temperature difference between either the feed line and the return line, or between one of said lines and the ambient or another heat sink, respectively.
2. Discussion of the Art
Known thermal power measuring devices and heat energy meters, having movable elements for measuring the mass flow of media, wear out and are sensitive to impurities. Measuring devices without movable elements become inexact when sediments accumulate, when flow conditions alter and become turbulent and when the medium is a polyphase one. Said meters and measuring devices are not appropriate for small flows and they are only applicable for a declared medium. It is therefore an object of the invention to provide a thermal power meter and/or a heat energy measuring device, both without movable and wearable elements, and appropriate for small flows, which further are independent of accumulation of sediment and impurities, of turbulence and of the circumstance that the medium is a polyphase one, as well as applicable for various media.
It is a demanding problem to accurately measure the thermal power exchanged in a heating device by means of a liquid or gaseous medium. The mass flow and the temperature difference (T.sub.inlet -T.sub.outlet) has to be continuously measured, and one has to know the average thermal capacity, C.sub.p, of the medium with this temperature region, i.e. ##EQU1## in order that the basic equation for exchanged thermal power be electronically or in another manner calculated, as follows: ##EQU2## Contrary to the circumstance that it is, according to the present state of the art, not difficult to accurately measure the temperature differences, .DELTA.T, particularly by differential connection of sensors, continuous measurement of the mass flow, m, is very demanding in practice. Consequently, rather, volume flows are measured with consideration of the density of medium. However, if the medium is polluted, compressible, a two-phase medium (fluid plus gas bubbles or gas+droplets), or a pulsating medium, the volume flows cannot be accurately measured with ease. Measurement of smaller flows in large dimension tubes, where the velocities are low and alterable according to the cross-section, represent a special problem.
Direct volume flow meters, e.g., gasometers, are not appropriate for continuous flows, particularly when the flows are not steady ones. Indirectly measuring volume gauges, based on various wheels (Voltman's wheel), sprockets, elliptical sprockets, rings, vane runners, flaps and the like, are sensitive to impurities (axles and journals). Namely, due to impurities, sediments, corrosion and wearing-out, the clearance between the movable and the stationary elements diminishes or enlarges. Consequently the accuracy declared is reduced by the use of such gauges. The later are neither applicable for compressible media, two-phase media or at faster flow alterations.
According to the prior art, many indirectly measuring volume-flow gauges exist which measure indirectly twice, i.e., they measure a value dependent upon the velocity, the latter being dependent upon both the volume flow and the cross-section. The velocity is not constant, but varies according to the cross-section, which essentially influences the exactness of these methods. Velocity measurements by means of pressure differences at restrictors, Venturi tubes, in tube bends, etc. are of this kind. Instead of using pressure differences, the velocity is also measured by comparing the heat transmission, e.g., by means of a hot wire or a heated plug. The disadvantage of the latter methods lies in that the response is dependent upon the sensor, and further dependent upon the impurities, the temperature, turbulence, bubbles, etc., which all reduces the reliability and the accuracy of the measurements.
Lately, electronics has enabled several indirect velocity measurements, e.g., by using the Doppler effect by means of ultrasonics or laser; the measurement of passing time of Karman vortexes or of bubbles within a tube; the compensation/comparison method relating to the coefficient of heat transmission in combination with the Venturi tube; magnetostriction and induction methods; as well as axial-ionisation or radialionisation methods. Most of the measurements mentioned above require composite electronics and are only accurate under restricted, declared conditions.
To measure the thermal power, Q, the gauges must also consider the thermal capacity, C.sub.p, of the medium. Said thermal capacity depends upon the material, the temperature and upon the pressure as well. When computing (automatically or non-automatically) the thermal power or the heat energy exchanged within a period, mostly the density of the medium has to be considered in the measurements described above. Therefore, the errors sum up rapidly.